PRESSURE OF LIGHT IN ASTRONOMY 73 



assumption that the tail was gaseous. But this is 

 just a case in which light pressure cannot account 

 for the formation of a tail at all, as indeed Fitz- 

 gerald recognized later. 1 For no gas will absorb 

 from sunlight enough momentum to acquire the 

 motion observed in the stretching of the tail. 2 



Soon after Fitzgerald's suggestion Lebedew 3 

 investigated the pressure on small absorbing 

 particles, and showed that if the particles were 

 small enough, repulsion would overcome attraction, 

 and that the motions observed in some comets' 

 tails could be thus accounted for, on the supposi- 

 tion that they consisted of particles of the requisite 

 smallness. Arrhenius 4 discussed the question in 

 greater detail, and sought to explain the self- 

 luminosity of the tail, which certainly coexists 

 with light reflected from the sun, by certain electrical 

 actions emanating from the sun. 



Observation shows that the head of a comet on 

 the side facing the sun presents the appearance it 

 may be only an appearance of jets of matter spout- 

 ing forward in various directions with nearly the 

 same velocity, and then turning round and stream- 

 ing out behind very much as a fountain spouts up 

 drops which rise a little way and then stream 



1 Scientific Writings, p. 531. 2 Note 6, p. 92. 



3 Ann. der Phys. u. Chem., XLV., 1892. 



4 Lehrbuch der Komuschen, Physik, 1903, or Worlds in the 

 Baking (1908), chap, iv, 



